At present, CFC-12 is mainly used as a refrigerant for car air conditioners and refrigerators. However, development of a refrigerant which can be used as a substitution for CFC-12 has been desired with a viewpoint of protection of the ozone layer.
HFC-134a as a refrigerant has properties similar to those of CFC-12, and it can be used as a substitute for CFC-12 with only minor changes of equipment being necessary. Likewise, HFC-134 (1,1,2,2-tetrafluoroethane), which is an isomer of HFC-134a, can also be used.
In a refrigeration system using CFC-12, mineral oil is used as a lubricant for a compressor. CFC-12 is miscible with mineral oil over a wide temperature range and therefore, even in the refrigeration system where evaporation and condensation of the refrigerant are repeated, phase separation of the refrigerant from the lubricant does not occur.
However, HFC-134a is not satisfactorily miscible with mineral oil. Therefore, when mineral oil is used, the mineral oil is replaced by the refrigerant, for example, in a compressor, causing various serious problems. For example, the lubrication becomes unsatisfactory and the lubricant adheres to the inner wall of a heat exchanger, leading to a lowering of the heat exchange efficiency.
The lubricant for a refrigerator using HFC-134a as the refrigerant should be miscible with HFC-134a at least over a temperature range of from 0.degree. to 50.degree. C., preferably over a wide temperature range of from -20.degree. to 70.degree. C., more preferably over a wider temperature range of from -40.degree. to 90.degree. C., and most preferably over a still wider temperature range.
The lubricant should have a kinetic viscosity of from 3 to 500 centistokes (hereinafter, frequently abbreviated as "cst") at 40.degree. C., preferably from 5 to 300 cst at 40.degree. C., more preferably from 5 to 170 cst at 40.degree. C., and most preferably form 10 to 150 cst at 40.degree. C., for exerting excellent lubricating performances.
Accordingly, development of a lubricant having a desired viscosity and being miscible with HFC-134a over a wide temperature range has been desired.
Various polyoxyalkylene glycol substances have been proposed as the lubricant to be used in combination with HFC-134a. Particularly, a polyoxyalkylene glycol having at least two hydroxyl groups (specifically, polyoxypropylene glycol), disclosed in the specification of U.S. Pat. No. 4,755,316, is taught to exhibit a good miscibility with HFC-134a over a wide temperature range. However, the temperature range over which this lubricant is miscible with HFC-134a is still unsatisfactory, and improvement of the miscibility, especially at high temperatures, is required.
Polyoxyalkylene glycols have not only unsatisfactory lubrication properties under application conditions, but also high moisture absorption properties and therefore, various problems are likely to arise with respect to, for example, the freezing of water, corrosion of metals, and lowering of the volume resistivity (such a lowering of the volume resistivity causes a problem in the case of a closed type freezer, such as a refrigerator). Accordingly, polyoxyalkylene glycols are not an excellent lubricant for a refrigeration system from a practical point of view.
A perfluoropolyether oil appears to be a lubricant miscible with HFC-134a which is a fluorine-containing compound.
Various perfluoroether oils having different structures can be mentioned. For example, there can be mentioned oils comprised mainly of recurring units, which may be either of a single type or of a plurality of types, represented by the following formula (V): ##STR2## wherein n' is 1, 2 or 3 with the proviso that n' is not simultaneously 1 with respect to all of the recurring units of the perfluoroether portion.
Specific examples of perfluoroether oils include those, which are available in the market as a vacuum pump oil and a lubricating oil, having a terminal stabilized with a perfluoroalkyl group, as shown below: ##STR3## wherein q.sub.1, q.sub.2, q.sub.3, q.sub.4, q.sub.5 and q.sub.6 are each a positive integer.
The present inventors examined the miscibility of these various perfluoropolyether oils with HFC-134a, and found that each oil shows a good miscibility with HFC-134a at temperatures higher than about room temperature, but oils, except those having a low molecular weight, are unsatisfactory in the miscibility with HFC-134a at low temperatures below 0.degree. C. Accordingly, it was confirmed that these oils are not suitable as a lubricant for a refrigeration system employing HFC-134a as the refrigerant.
In Japanese Unexamined Patent Application Publication No. 60-96684, it is taught that when a fluorolubricant, such as a fluorinated silicone or a perfluoropolyether, is used in a fluorocarbon motive fluid for a heat pump, the heat resistance of a fluorocarbon refrigerant is improved. However, no description is made with respect to the miscibility of a tetrafluoroethane with a fluoro-lubricant. Japanese Unexamined Patent Application Publication No. 1-118598 teaches that a perfluoropolyether and/or a fluorinated silicone can be used as a lubricant for fluorocarbon refrigerants. However, with respect to the miscibility at low temperatures below about room temperature, no description is made.
In Japanese Unexamined Patent Publication Application No. 62-146996, it is taught that addition of up to 5% by weight of a carboxyl group- or hydroxyl group-containing perfluoropolyether derivative as an extreme pressure additive to a lubricant is effective. However, no description is made with respect to the miscibility of this carboxyl group- or hydroxyl group-containing perfluoropolyether derivative with a fluorocarbon refrigerator, such as a tetrafluoroethane.
In Japanese Examined Patent Application Publication No. 51-2083 and the specification of U.S. Pat. No. 3,654,273, it is taught that a perfluoropolyether type triazine compound can be used as a lubricant, but no description is made with respect to the miscibility of this compound with a fluorocarbon refrigerant, such as a tetrafluoroethane. The lubricating performances of a perfluoropolyether type triazine compound and poly(hexafluoropropylene oxide) are described in Internationales Jahrbuch der Tribologie (International Yearbook of Tribology), 1, 383 (1982), but the miscibility properties of these compounds with a fluorocarbon refrigerant, such as a tetrafluoroethane, are not described at all.
In these situations, the present inventors have made researches with a view toward developing a substance showing not only a good miscibility with a tetrafluoroethane, such as HFC-134a, over a wide temperature range of from low temperatures to high temperatures, but also a viscosity ensuring satisfactory lubricating performances. As a result, it has been found that a fluorine-containing compound having a specific viscosity and having a structure represented by formula (I) defined herein or a composition comprising at least 25% by weight of this fluorine-containing compound and the balance of other oil, has not only a good miscibility with a tetrafluoroethane, such as HFC-134a but also a viscosity suitable for a lubricant for a refrigeration system and, is therefore suitable as a lubricant for use in a refrigeration system using a refrigerant comprising a tetrafluoroethane, such as HFC-134a. The present invention has now been completed, based on this finding.
It is therefore a primary object of the present invention to provide a novel lubricant for use in a refrigeration system, which exhibits not only a good miscibility with a tetrafluoroethane, such as HFC-134A which is a refrigerant promising as a substitute for CFC-12, over a wide temperature range of from low temperatures to high temperatures, but has also a viscosity suitable for a lubricant for use in a refrigeration system.
Another object of the present invention is to provide a refrigerant composition comprising the above-mentioned lubricant for use in a refrigeration system and a tetrafluoroethane refrigerant.
These and other objects, characteristic features and advantages of the present invention will become apparent from the following detailed description and the appended claims.